Optimization and application of shock wave measurement technology for shock-timing experiments of small-scale capsules

Abstract

In the realm of laser fusion research, the precision of shock-timing technology is pivotal for attaining optimal adiabat tuning during the compression phase of fusion capsules, which is crucial for ensuring the high-performance implosion. The current main technological approach for shock-timing experiments is the use of keyhole targets and VISAR diagnostics to measure the shock velocity history. Nonetheless, this approach encounters limitations when scaling down to smaller capsules, primarily due to the reduced effective reflection area available for VISAR diagnostics. This study introduces a novel high-precision shock-timing experimental methodology for a double-step radiation-driven implosion with a 0.375mm radius capsule on a 100 kJ laser facility. By developing a theoretical framework for calculating the intensity of VISAR images with spherical reflective surfaces, an innovative experimental technical route is proposed to utilize the keyhole cone reflection effect to enhance the VISAR diagnostic spatial area, effectively increasing the effective data collection region by nearly threefold for small-scale capsules. The technique has been adeptly applied to measure shock waves in cryogenic liquid-deuterium-filled capsules under shaped implosion experimental conditions, obtaining high-precision shock-timing experimental data. Experimental data reveals that the application of this technology has markedly enhanced both the image quality and the precision of data analysis for shock wave velocity measurements in small-scale capsules. Furthermore, it has been discovered that under similar laser conditions, there exist considerable variations in the shock velocity profiles. Simulation analysis suggests that the differences in the "N+1" reflected shock wave's catching-up behavior, caused by minor variations in laser intensity, are the main reason for the substantial merge velocity differences. It is demonstrated that minor variations in laser parameters can significantly affect the transmission behavior of the shock wave. This experiment highlights the intricate sensitivity of shock wave transmission in the high-performance shaped implosion physics process at the current small capsule scale, and it is essential to conduct shock-timing experiments for precisely tuning the actual shock wave behavior. This research not only lays a robust technical foundation for the advancement of adiabat tuning experiments on China's 100 kJ laser facility but also carries profound implications for the ultra-high pressure physics research based on the spherical convergence effect.